But more important than merely being an "address" for a book, call numbers identify the primary subject of a book, thus making it possible for books on the same subject to be grouped together.

Call numbers are derived from the letters and numbers of the classification system being used by a particular library, either Dewey Decimal or Library of Congress. Since CSM Library uses the Library of Congress system, let's examine it in more depth.

LIBRARY OF CONGRESS CALL NUMBERSLibrary of Congress call numbers are made up of at least three lines:

The first line is a letter or letters arranged alphabetically.

The second line is a whole number arranged numerically.

The third line is a combination of a letter and number arranged alphabetically and then numerically.

Sometimes there is an additional line with a letter-number combination (which is arranged in the same way as the third line), and sometimes there is a publication year on the bottom line of the call number.

For example, here is a typical LC call number:

QB

44

M65

To find the above book with this call number on the shelf, you would first look for the shelves with books with call numbers starting with Q. Books are arranged on the shelves by call number beginnning with books with call numbers starting with A and ending with books with call numbers starting with Z. At the end of each set of shelves are labels identifying the call numbers of the books on those shelves.

The order of different call numbers is determined by comparing each line of each call number starting from the top and then going down until you find a difference between them. If the first lines of two call numbers are the same, check the second lines. If both the first and second lines are the same, compare the third lines, and so on.

Continuing with our example, once you find the books with call numbers starting with Q, follow the call numbers of the books and you will then see books with call numbers starting with QA and then books with call numbers starting with QB. Thus, call numbers are arranged first by the letter or letters on the first line in alphabetical order.

Note: A decimal point may be included at the beginning of the third line (as in .H27, .M15 and .M3 below) but, whether or not a decimal point is included, the order is the same.

Q

771

.H37

Q

850

C125

QA

52

.M15

QA

418

.J32

QA

2645

L433

QB

44

M65

QC

227

F22

Let's continue to look for the call number in our example:

QB

44

M65

Once you find the call numbers starting with QB, look at the numbers on the second row of the call number. Follow them numerically until you find 44. The second line of an LC call number is arranged numerically.

Now you've found the books that start with:

QB

44

If there is only one book that has a call number beginning with those two lines, you don't have to look any farther to figure out the order. When you find many call numbers with the same first two lines, however, you need to compare the third lines of each one. Understanding the order of the third line, with the letter-number combination (M65, in our example) can be a little confusing. The combination is read first alphabetically by letter and then numerically by the number. The tricky part is that the number is a decimal number.

First find the letter (M, in our example) alphabetically. If all of the letters are different, the order is decided just by the letters. If there are any call numbers with the same first two lines and with the same letter at the beginning of the third line, you then have to compare the numbers following the letter.

The number after the letter (65, in the example) is a decimal number. This is where many people get confused. Look closely at the following call numbers which are in correct order:

QB

44

.H37

QB

44

M125

QB

44

.M15

QB

44

.M3

QB

44

M6188

QB

44

M65

QB

44

R22

If you remember that the numbers in the third row after the letter are decimal numbers, the order will make sense. If you have trouble with decimal numbers, remember that there are imaginary zeros at the end of all decimal numbers. So in the example, you can see why M125 comes before M15 if you put a zero after the 15. Another way to understand this is to look at one number at a time. For example, the first number in both M125 and M15 is 1, but the second number in M125, which is 2, comes before the second number in M15, which is 5.

A decimal point may be included at the beginning of the third line (as in .H27, .M15 and .M3 above) but, whether or not a decimal point is included, the order is the same.

If there is a fourth line with a letter-number combination (and all previous lines of multiple call numbers are the same), that line would be arranged in the same way as the third line. For example:

QB

44

M3

R2485

QB

44

M3

R535

QB

44

M3

R59

Look at the following examples of correct call number order. Be sure that you understand why they are in the order shown. If you do not understand, be sure to ask a librarian to explain it to you.